Synergistic Enhancement of Flame Retardancy and Mechanical Properties in Vinyl Ester Resins via <scp>DOPO</scp> ‐Based Flexible Chain Grafting Modification

Vinyl ester resin (VER) is highly flammable, which limits its use in applications requiring high flame resistance. However, conventional flame‐retardant approaches often degrade its mechanical properties. To improve both the flame retardancy and mechanical performance, this work developed a grafting strategy incorporating phosphorus‐containing flexible side chains. Three DOPO‐based diol derivatives with different chain lengths (DOPE, DOPB, and DOPH) were synthesized and then grafted onto the VER backbone through 4,4′‐methylenediphenyl diisocyanate (MDI) to obtain VER/M‐DOPx systems. The modified resins showed obvious improvements in both flame retardancy and mechanical properties. VER/M‐DOPE and VER/M‐DOPB achieved UL‐94V‐0 ratings, with LOI values of 28.6% and 26.7%, respectively, while VER/M‐DOPH reached a V‐1 rating and an LOI of 28.9%. VER/M‐DOPB showed the best flame‐retardant effect, reducing the peak heat release rate (PHRR) and total heat release (THR) by 68.5% and 43.7% compared with VER. The incorporation of flexible DOPO‐based diol chains also enhanced toughness and strength; VER/M‐DOPE exhibited increases of 4.53%, 26.24%, and 81.35% in tensile, flexural, and impact strengths, respectively. These results indicate the feasibility of enhancing both the flame retardancy and the mechanical properties of VER through grafting phosphorus‐containing flexible side chains.